Re-open /dev/null inside the container

[adrianreber]

To use checkpoint/restore without --console-socket for testing it is necessary in runc/crun to start the container with &> /dev/null < /dev/null. This however results in using /dev/null from the host and not from the container. In this case stdin, stdout and stderr needs to be connected to /dev/null in the container. I did this for crun: containers/crun@bbb1fa9

Please also add this to youki.